Assessing the Immunogenicity of Intramuscular Inactivated Poliovirus Vaccine Against Type 2 With Bivalent Oral Poliovirus Vaccine in Routine Immunization

Oral poliovirus vaccines are live attenuated viral vaccines and the vaccine virus in OPV can mutate and acquire neurovirulence causing paralysis either due to vaccine-associated paralytic polio (VAPP) or due to circulating vaccine-derived polioviruses (cVDPVs), in which the attenuated vaccine virus not only acquires the ability to cause paralysis but can also circulate similar to wild poliovirus (WPV). The potential of vaccine virus to acquire neurovirulence and cause paralytic poliomyelitis is incompatible with eradication of polio. Therefore, polio eradication will require eventual cessation of all OPVs.

The last case of WPV2 was reported in 1999 in India. Type 2 vaccine virus in tOPV is most likely to cause cVDPVs and over 80% of cVDPVs over the past decade have been cVDPV2. Of the estimated 250-500 annual VAPP cases almost 40% are due to type 2. Thus, with the eradication of WPV2 it is imperative to prioritize removal of type 2 containing OPV.

The licensing and availability of bivalent OPV (bOPV) containing type 1 and 3 offers the option of an OPV that does not contain OPV2. In April 2013, the Strategic Advisory Group of Experts on Immunization (SAGE) recommended phased cessation of OPV types starting with the switch from trivalent oral poliovirus vaccine (tOPV) to bivalent OPV (bOPV). In November 2013, SAGE recommended that countries that introduce at least one dose of IPV in the routine immunization program should administer the first dose of IPV at ≥14 weeks of age. In countries with routine immunization schedule of 6, 10 and 14 weeks of age or 2, 3 and 4 months of age, IPV would then be added to the DTP3 visit for children on schedule or administered at the first immunization visit at 14 weeks or later if children are off schedule. For countries with 2, 4 and 6 month schedule, IPV could be added either to DTP2 or DTP3 visit. SAGE also proposed that countries have the flexibility to consider alternative schedules including administering IPV earlier than 14 weeks of age or administer more than one dose of IPV.

The principal objective of pre-eradication introduction of IPV is to mitigate the risk associated with an increased susceptibility to type 2 polioviruses when bOPV is introduced. Therefore, the objective is to achieve the highest possible type 2 population immunity with IPV, which is a product of the per dose immunogenicity of IPV and the coverage achieved by IPV at that vaccination visit in routine immunization.

At present, no study has assessed immunological response against type 2 poliovirus of a single dose of IPV at 14 weeks of age, the SAGE recommended target age for IPV introduction. Furthermore, there is need to determine the immunological response with other potential alternatives for IPV introduction in routine immunization within the context of EPI schedule including immunological response with more than one dose of IPV. Also there is urgent need to document the immunogenicity of bOPV at 6, 10 and 14 weeks of age with IPV against type 1 and 3 polioviruses.